1. Field of the Invention
The present invention relates to an oxygen absorbent composition, and more specifically, it relates to an oxygen absorbent composition which can be used to preserve cultural assets, for example, wooden sculptures such as lacquered containers and statues, painted plates, wooden buildings, horns such as carved horns, industrial art objects using bones, fabrics and dyes fabrics of silk, cotton, hemp and the like, paper materials such as secondhand books, second-hand documents, Japanese papers, books, sacred books, documents and scrolls, paintings such as oil paintings, watercolor paintings, Japanese paintings and calligraphic works, ambers, various dyes and pigments, specimens of insects and plants, leathers, photographs such as photo-graphic negative films, printed photographs, microfilms and slides, and strings of musical instruments which are made of organic materials and which tend to bring about strength deterioration, discoloration and fade-out by oxidation and which tend to suffer from damage by insects such as grain weevils.
2. Description of the Related Art
Cultural assets made of organic materials are desirably preserved at a medial humidity and under conditions that the humidity scarcely changes. Therefore, it has usually been carried out to preserve the cultural assets in a room or a warehouse in which a temperature is adjusted to about 20 to 25.degree. C. and a relative humidity is adjusted to about 30 to 50% mechanically by an air-conditioning equipment. In addition, it has also be done to further positively keep the humidity at a constant level by the use of a humectant and a humidifying box. In any case, however, oxygen is not removed from a preservative environment, and hence, the value of the cultural assets has actually been lost owing to discoloration and deterioration by reactions with oxygen and owing to the propagation of microorganisms such as molds. Moreover, fumigation has also been employed for the killing of insects by the use of methyl bromide, but the warehouse is stuffy with methyl bromide for a long time, which is definitely harmful to operators.
Furthermore, in a certain case, an oxygen absorbent has been utilized for the preservation of the cultural assets made of the organic materials. In the conventional oxygen absorbents, however, there is used, as an oxidizable main component, a composition comprising iron or an iron powder such as iron carbide and an electrolyte such as a metal halide, a reducing inorganic salt such as a sulfite, a thiosulfate or a ferrous salt, a polyphenol such as hydroquinone, catechol, resorcin or pyrogallol, a reducing saccharide such as glucose, a reducing polyhydric alcohol such as ascorbic acid or erysorbic acid, or a reducing agent such as glycerin. However, all of them require moisture in an oxygen absorbing reaction, and the oxygen absorbent composition itself contains the moisture. Hence, the humidity in a closed container rises, and in consequence, the moisture is inconveniently given to the articles to be preserved. When the oxygen absorbent composition itself does not contain the moisture, the moisture is taken from the closed atmosphere, and an oxygen absorption rate is affected by the humidity of the closed atmosphere, so that the oxygen absorption rate becomes slow. For these reasons, the performance of the oxygen absorbent composition inconveniently becomes unstable.
As a technique for removing oxygen while the humidity is maintained at a medial level, Japanese Patent Application Laid-Open No. 96060/1992 discloses a method in which a gas barrier bag is filled with an oxygen absorbent composition comprising an unsaturated aliphatic compound, an oxygen absorbent comprising a substance for accelerating an oxygen absorption and a humectant, and it is used for the preservation of photographs. In addition, Japanese Patent Application Laid-Open No. 100537/1992 discloses a method in which secondhand books, secondhand documents, magnetic tapes and drugs are preserved by such a method. The other similar methods are also disclosed in U.S. Pat. No. 5,286,407 and Japanese Patent Application Laid-Open Nos. 6287/1992, 29741/1992, 87961/1992, 239092/1992 and 268085/1992. In addition, it is disclosed in U.S. Pat. No. 4,908,151 and Japanese Patent Application Laid-Open Nos. 198962/1988 and 67252/1989 that dry foods and drugs are preserved and metallic products are prevented from rusting by the use of an oxygen absorbent composition comprising an unsaturated fatty acid, a transition metal, a basic substance and an oxygen absorbent.
However, it has been elucidated that when the oxygen absorbent compositions which are employed in these publications are used, formaldehyde and the like are generated by the oxygen absorbing reaction, as described in comparative examples which will be given hereinafter in this specification. For example, if formaldehyde is present, litharge (lead monoxide) which is a kind of pigment is reduced, so that yellow color is changed into white color. As understood from this example, formaldehyde and the like generated during the oxygen absorption are reactive, and they discolor some kinds of dyes and pigments. Accordingly, the oxygen absorbent compositions disclosed in these publications are scarcely useful as the oxygen absorbents for the preservation of the cultural assets made of the organic materials.
Furthermore, in these publications, perlites, activated carbons and zeolites are enumerated as adsorbents in the oxygen absorbent compositions. Moreover, as materials for absorbing substances produced by decomposition during the oxygen absorption of the unsaturated aliphatic compound, there are exemplified basic substances such as amine compounds, magnesium oxide, calcium oxide and calcium hydroxide. However, even if the perlite or the zeolite is used to adsorb a reactive gas such as formaldehyde, in the medial humidity atmosphere, adsorption sites are first covered with molecules of the moisture, so that the reactive gas cannot effectively be adsorbed. The activated carbon effectively functions even in the relatively medial humidity atmosphere, but its ability to adsorb formaldehyde is low. Therefore, a large amount of the activated carbon is disadvantageously required. On the other hand, magnesium oxide, calcium oxide and calcium hydroxide cannot effectively absorb the reactive gas, as shown in the undermentioned comparative examples. Additionally, in these publications, the amine compounds are enumerated as the basic substances, but the suitable kinds of amine compounds are not concretely exemplified at all. When some amine compounds were merely used as they are by the inventors of the present application, any effect was not observed.
Furthermore, since these disclosed techniques are used in the medial humidity atmosphere, they suggest the employment of the humectant. However, if the humectant is actually used, the volume of the oxygen absorbent composition itself correspondingly increases. Thus, if no use of the humectant is allowed, such a constitution is preferable. In the above publications, examples of a carrier for the oxidizable main component include papers, unwoven fabrics, perlites, activated carbons and zeolites, but when the oxidizable main component is supported on the paper and the unwoven fabric, they becomes sticky, and hence they are not practical. When the oxidizable main component is supported on the activated carbon, its reactivity with oxygen is too high, so that deactivation is liable to occur in air. In addition, a large quantity of heat is generated, and for these reasons, the handling of the activated carbon is inconvenient and hence it is not preferable. The perlite and the zeolite themselves have a moisture absorption ability, and so they decrease the humidity in the closed system. Hence, the humectant must be used unavoidably.